Neutron generating apparatus having a target containing both deuterium and tritium gases



W. R. ARNOLD Get, 31, i967 3,350,563 AINING NEUTRON GENERATING APPARATUSHAVING A TARGET CONT I BOTH DEU Origlnal Filed March 8, 1954 TERIUM ANDTRITIUM GASES 4 Sheets-Sheet l Oct. 3l, 1967 w. R. ARNOLD 3,350,563

NEUTRON GENERATING APPARATUS HAVING A TARGET CONTAINING BOTH DEUTERIUMAND TRITIUM GASES Original Filed March 8, 1954 4 Sheets-Sheet 5 W CU Uc.31, 1967' wpR. ARNOLD 3,350,563

NEUTRON GENERATING APPARATUS HAVING A TARGET CONTA'INING BOTH DEUTERIUMAND TRITIUM GASES Original Filed March 8, 1954 A United States Patent ONEUTRON GENERATING APPARATUS HAVNG A TARGET CONTAINING BOTH DEUTERIUMAND TRITIUM GASES Wayne R. Arnold, deceased, late of Ridgefield, Conn.,by Marian H. Arnold, executr'nr, Albuquerque, N. Mex., assignor, bymesne assignments, to Schlumberger Tech- 'lology Corporation, Houston,Tex., a corporation of exas Application Sept. 8, 1958, Ser. No. 763,154,now Patent No. 3,082,326, dated Mar. 19, 1963, which is a division ofapplication Ser. No. 414,761, Mar. 8, 1954, now Patent No. 2,914,677,dated Nov. 24, 1959. Divided and this application Mar. 8, 1963, Ser. No.263,997

4 Claims. (Cl. Z50-84.5)

This invention rela-tes to methods and apparatus for well logging and,more particularly, pertains to a new and improved neutron generatorespecially adapted to tra- Verse the narrow contines of `a well or borehole, although useful in a variety of other applications. Since aneutron generator embodying :the invention is ideally suited to theneeds of well logging service, it will be described in that connection.

This application is a division of copending applicaxtion Ser. No.763,154, filed Sept. 8, 1958, which was issued Mar. 19, 1963, as PatentNo. 3,082,326, and, in turn, was copending with and a division ofapplication Ser. No. 414,761, tiled Mar. 8, 1954, now Patent No.2,914,677, issued Nov. 24, 1959.

It has been proposed heretofore that =a generator of high energyneutrons be employed in neutron-gamma -ray or in neutron-neutronlogging. As contrasted with a radium-beryllium source conventionallyutilized for such logging, a neutron generator may feature a negligible:amount of radiation other than the desired neutrons, Ia higher yield ofneutrons, a controlled yield of neutrons, neutrons at higher energiesthan formerly possible, monoenergetic neutrons and control of thegenerator so as to permit its deactivation `prior to withdrawal from aWell. The iirst five of these attributes are important in obtaining moreinformative logs, while the last is valuable in minimizing healthhazards to operation personnel.

In general, prior neutron generators were only suited for laboratory useand were not designed to meet the severe requirements imposed on welllogging equipment. Thus, presently available neutron generators are muchtoo large to be passed through a bore hole. The components `are notadaptable to the source-detector spacing requirements of well logging.Moreover, these neutron generators are too critical in their operationand too frangible for logging service.

It is. therefore, a primary object of the present invention to providean improved neutron generator which meets all requirements of loggingservice.

A specific obiect of the present invention is to provide an improvedneutron generator which has a small enough diameter to permit itsintroduction into an inherently cylindrical bore hole.

Another object of the present invention is to provide `an improvedneutron generator which may be reliably operated during a logging runwithout requiring critical and continuous operating Iadjustments.

An additional object of the present invention is to provide an improvedneutron generator which may be reliably operated at the high ambienttemperatures encountered at depth in logging operations.

Yet another object of the present invention is to provide an improvedneutron generator that is rugged enough to operate eiciently andreliably although subjected to the severe physical `shocks usuallyimposed on logging apparatus during transport to -and from a welllocation, as well as during a logging run.

3,356,563 Patented Oct. 31, 1957 A further object of the presentinvention is .to provide a novel method of well logging in which theimproved neutron generator may be employed and featuring more accuratequantitative data regarding the earth formations than heretoforeattainable.

These and other objects of the present invention are obtained byproviding a neutron generator comprised of an ion source, an ionIaccelerator and a target which preferably may be an element of theaccelerator. The target includes a substance adapted to react withbombarding ions of suicient velocity .to produce neutrons.

More specically, the generator comprises an envelope containing a gassuch as deuterium. A radiotrequency field is employed to excite .the gasand cause ionization in the ion source. A probe of the accelerator,which ef- Ifectively reaches into the region of the ion source, removespositive ions frorn the source and a suitably high potential differenceis impressed between the probe and the target so that these positiveions are accelerated to the required high veloci-ty prior to strikingthe target. The target includes Ia material containing tritium, an iso-'tope of hydrogen. From the resulting deuterium-tritium reaction,neutrons are derived.

In order to maintain the neutron ilux emanating from the target withinprescribed limits, a detector-integrator may lbe employed -t-o derive acontrol potential Vrepresenting -a characteristic of the neutron flux,such as the num- -ber of neutrons counted per unit time. This potentialis employed to ladjust the potential applied to the accelerating gapthereby effecting autornatic control of the neutron output since theyield is dependent upon the energy of the positive ions incident on thetritium target. y

The apparatus may further include a pressure-control system formaintaining the pressure of deuterium gas in the ion source at apreselected value desipte the fact that ions of the gas are continuouslywithdrawn. For this purpose, a pressure transducer is associated withthe ion source for deriving a pressure-control potential representinggas pressure. This potentialy automatically adjusts the amount of gasissuing from a deuterium supply associated with the ion source.

The pressure transducer may, for example, comprise a cathode and ananode exposed to the gas of the ion source. A magnet is employed toprovide a magnetic eld component for effectively increasing the path forelectrons traveling between the cathode and anode. Thus, a continuousionic discharge occurs. The resulting anode-cathode current is dependentupon the pressure of the gas, and from this current, the aforementionedpressure control potential is derived.

Because the neutron generator is a closed contiguous system, it isnecessary to balance two opposing requirements. The gas pressure in theion source and pressure gauge must be high enough to allow sufficientionization to be produced in each to give adequate ion currents fortheir operation. However, the gas pressure must be low enough to avoidappreciable production of ionization in the accelerating gap. A stablebalance may be achieved by constructing these three components so thatthe paths traveled by electrons relative to their mean-free-paths in theion source and pressure gauge are long, whereas their paths arel shortrelative to the mean-free-path for electrons in the accelerating gap.

As used herein, the term mean-free-path denotes the average distancethat electrons travel in a particular gas between collisions with atomsor ions of that gas. If a sufficient number of such collisions takeplace, the ionization produced is cumulative, resulting in a continuousionic discharge.

The spacing between electrodes of the accelerating gap is made smallenough to minimize the path travelled by electrons in this region. Inthis way, ionization is inhibited despite the extremely highacceleration potential applied to the gap. The spacings betweenelectrodes in the ion source and pressure gauge are made large tomaximize the path of travel of electrons and thereby assure theoccurrence of strong ionization. In addition, an auxiliary magneticfield may be utilized to cause the electrons to describe non-linearpaths, such as spirals, in order to extend effectively the path oftravel.

The novel method of well logging in accordance with the presentinvention makes use'of the automatic output control system of theneutron generator. However, the detector-integrator may be arranged torespond either to the emitted fast neutrons or to neutrons which wereemitted as fast neutrons but have been slowed to low, or thermal,energies. By controlling the flux of emitted neutrons, in the first ofthese prescribed methods, indications of other nuclear phenomena, suchas captured gamma radiation resulting from the bombardment of thesurrounding earth formations by neutrons, provides one type ofinformative log. On the other hand, by controlling the flux of emittedneutrons in response to neutrons which have been slowed, and record-ingthe resulting gamma radiation, another type of log of the formations maybe obtained.

The novel features of the present invention are set forth withparticularity in the appended claims. The present invention, both as toits organization and manner of operation, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in connection with the accompanying drawingsin which:

FIGS. 1A, 1B and 1C illustnate schematically the upper, middle and lowerportions, respectively, of neutron well logging apparatus embodying thepresent invention;

FIG. 2 is an enlarged view in longitudinal section of a portion of FIG.1B;

FIGS. 3 and 4 are views in longitudinal section of modifications whichmay be made to respective portions of the structure of FIG. 2;

FIG. Sis aschematic diagram of a high voltage power supply suitable foruse in the portion of the apparatus shown in FIG. 1C;

FIG. 6 is a view in longitudinal section of the power supply representedin FIG. 5, as incorporated in logging apparatus;

FIG. 7 is a sectional View taken along line 7--7 of FIG. 6; fand FIG. 8represents a modification which may be made to the circuit arrangementof FIG. 5.

In FIG. 1A of the drawings, the neutron well logging apparatus embodyingthe present invention is shown disposed in a bore hole 10 traversing aplurality of earth formations 11. Bore hole 10 usually contains ahydrogenous drilling liquid 12, such as a water base or oil base mud,and it may be lined with oneV or more strings of metallic casing (notshown) Vor it may be uncased as illustrated.

The neutron well logging apparatus may comprise a pressure-resistanthousing 13 enclosing a neutron generator 14 (FIG. 1B), a radioactivityresponsive device 15 (FIG. 1C) for detecting the phenomena to beobserved, and associated electronic equipment required for properoperation 'of the neutron generator and the detector, as described ingreater detail hereinafter.

A shieldplate 16, disposed above detector 15, may be employed to shieldthe detector from radiation emanating from generator 14. If theapparatus is to be used for obtaining neutron-gamma ray logs, the shieldmay be composed'of lead, and if neutron-neutron logs are desired, theshield may be constructed of a boron compound such as boron carbideembedded in paraffin. Of course, a composite shield of lead andboron-loaded paraffin rnay be utilized if both types of logs are to bemade with the equipment, either successively or simultaneously.

Housing 13 is suspendedin the bore hole by means of an armored cable 17which, in connection with a winch (not shown) located at the surface ofthe earth, is

utilized to lower and raise the apparatus in the bore hole in thecustomary manner. As will be described later in detail, cable 17comprises a plurality of insulated conductors that electrically connectthe apparatus within housing 13 with surface equipment 9.

The neutron generator 14 (FIG. 1B) is suitably supported by aconventional shock mounting (not shown) within housing 13. The generatorcomprises an evacuated envelope 18, preferably constructed of out-gassedPyrex glass, and filled with deuterium gas under a selected pressurewhich may be in the neighborhood of 1 to l0 microns of mercury.

As best seen in FIG. 2, a partition 19 formed by a disc of Pyrex glasseffectively divides the upper portion of envelope 18 into apressure-gauge section 20 (to be described more fully hereinafter) andan ion source section 21. These sections are in communication via acentral aperture 22 in partition 19.

A cylindrical support 23, constructed of thin glass, is cemented orotherwise secured at one of its ends to partition 19 and extends intoenvelope section 21 in essentially coaxial relation therewith. Supportedon the outer surface of cylinder 23 are a pair of spaced, annular bands24, 25 of conductive material. These bands form a discharge gap and eachis connected to a respective one of leads 26, 27 which extend throughthe wall of the envelope 18. These leads are suitably fused to theenvelope in a known manner, thereby forming pressuretight seals.

A cylindrical member 28 of magnetic material, such as an alloy ofaluminum, nickel and cobalt, commonly referred to as Alnico, receivesenvelope 18l and is positioned between the transverse planes defined byelectrodes 24 and 25. Member 28 is permanently magnetized so that itsends are of opposite magnetic polarity, thereby establishing an axialmagnetic field within envelope 1S.

The strength of this magnetic field is preselected, in view of thespacing between electrodes 24 and 25 to achieve a path of travelrelative to the mean-free-path for electrons, which move spirally due tothe field, sufiiciently long to promote strong ionization of the gas.For example, a strength of 400 gauss with an electrode spacing of 3inches has been found suitable.

To initiate ionization of the gas within ion source 21, a pellet 29 ofradioactive material, such as radium, is cemented, lor otherwise fixed,to the exterior surface of cylinder 23.

Support 23 terminates at, and is cemented to, a glass disc 30 slidablysupported at its periphery on the inner surface of envelope 18. Member30 has a central extension 31 of frusto-conical configuration projectinginto the confines of cylinder 23. The tip portion of extension 31 tlhsuscontains an opening 32 axially aligned with envelope A metal probeelectrode 33 of frusto-conical form, positioned in interfitting, spacedrelation with glass cone 31, is provided with an axially aligned opening34. The probe has an enlarged metal base portion 35 of generallytoro'idal form and an electrical connection may be made to the probe bymeans of a metallic lead 36 that passes through and is fused or sealedto the non-conducting wall of envelope 18.

Another metallic lead 37, also sealed to the envelope 18, passes througha radial opening 38 in base 35 of the probe to provide an electricalconnection to a metallic focus electrode 39 of frusto-conicalconfiguration. The focus electrode is positioned 'in interfitting,spaced relatzign with probe 33 and it has an axially aligned openingNeutron generator 14 further comprises a tubular target electrode 41positioned in axial alignment with respect to envelope l18. Thiselectrode preferably is constructed of a metal having a temperaturecoefiicient of expansion which corresponds to the glass in the envelope.For example, a low-expansion alloy of nickel, cobalt,

manganese and iron, commonly referred to as Kovar, may be employed. Thelower portion of the envelope is flared inwardly to form a reentranttubular section 42 in which the lower portion of electrode 41 isembedded and fused.

The target electrode extends to the vicinity of focus electrode 39 whereit is closed by an integral, hemispheric cap 43. The spacing between thecap 43 of the target and the probe 33 is arranged to be smaller than themean-freepath of electrons traversing envelope 18. Thus, ionization ofdeuterium gas does not occur when these electrodes are suitablyenergized to operate as an accelerating gap for the deuterium ions.

The cap preferably is-plated with a layer 44 of zirconium, or otherhydrogen-absorbing metal, which is hydrided in a well-known manner withtritium. For example, the zirconium layer may be alternately heated andcooled in an atmosphere of this isotope of hydrogen. Of course,deuterium may be employed for this purpose if a deuteriurn-deuteriumreaction is desired instead of a deuterium-tritium reaction.

A lead 45 extends through reentrant envelope section 42 into hollowelectrode 41 to which it is soldered or welded in order to provide anelectrical connection to the target.

To preclude strong ionization of the gas at the target end of generator14, an auxiliary member 46 is arranged to prevent the possibility ofelectrons traversing a gap more than a mean-free-path in length. Thismember is of generally toroidal form and has an axial opening 47receiving electrode member 41. It is secured to and electricallyconnected with the member 41 at a position such that its outer, curvedsurface is relatively closely spaced with respect to base 35 of theprobe. If desired, envelope 18 may be reduced in diameter in an areabetween members 35 and 46 so as to prevent the existence of a straightpath of any consequential longitudinal length along the inner surface ofthe envelope.

The upper, pressure-gauge section 20 of envelope 18 is provided with aconventional header 48 by means of which a plurality of leads 49-56 areintroduced through pressure-tight seals. A horseshoe type magnet 57 ofgenerally rectangular configuration is supported by leads 51 and 54 withits bight end 58 adjacent to partition 19. The magnet has pole faces 59and 60 facing each other and disposed on opposite sides of the plane ofa ringlike electrode 61 that is connected to the inner side of bight 58.'Ihe magnet 57 and electrode 61 constitute an anode for the pressuregauge.

The gauge also includes cathode plates 62 and 63 positioned parallel to,but slightly spaced from, individual ones of pole faces 59 and 60 andconnected to leads 52 and 53. Layers 64 and 65 of glass insulation areinterposed'between face 59 and plate 62 and face 60 and plate 63, andthese layers are supported by individual glass sleeves 66, 67 whichsurround and are fixed to the portions of leads 52 and 53 withinenvelope 18.

A baie plate 68, constructed of a refractory material of low atomicnumber, such as quartz of beryllium oxide, is supported on bight portion58 of magnet 57 in parallel, spaced relation with respect to partition19. It is arranged to intercept any particles moving through opening 22in a general direction from ion source 21 towards magnet 57.

Also supported within section 20 of envelope 18 by leads 49, 50 and 55,56 are filaments 69 and 70. Filament 69 is composed of a metallicdeuteride, such as zirconium deuteride, arranged so that the pressure ofdeuterium over its surface increases as the temperature increases.Filament 70, on the other hand, is composed of a getter material, suchas zirconium. Thus, at a selected temperature, gases are absorbed by thelatter filament. Alternative materials from which filaments 69 and 70may be constructed are titanium and uranium. To conserve power,individual, tubular heat shields 71, 72 of a reilective material of highmelting point, such as tantalum or molybdenum, enclose 6 filaments 69and 70 and are electrically connected -to filament leads 50 and 55.

In constructing generator 14, the usual precautions observed in thefabrication of discharge devices are observed. For example, metalmaterials for the various electrodes 24, 25, 33, 35, 39, 41, 46, 57, 61,62 and 63 are selected so that there is relative freedom from gases thatmay be absorbed prior to or during the fabrication process and which maybe later expelled in operation to contaminate the generator. Moreover,with the exception of the electrodes 61, 62 and 63 of pressure gauge 20which should be constructed of a metal that is a good secondary electronemitter, the electrode metals may be selected on the basis of lowsecondary-electron-emission characteristics to minimize the possibilityof breakdown. Alternatively, a readily outgassed metal may be employedif coated with a thin layer of material of low thermal emissivity. Theoriginal outgassing is accom plished via a conventional tube (not shown)which projects externally of header 48. The required amount of deuteriumis then introduced through the tube before it is sealed.

Referring now to FIG. 1A, in order to provide power for operatinggenerator 14 and its associated circuitry and yet remain within thevoltage and current-carrying capabilities of the conductors in cable 17,power is supplied by a three-phase alternating current source located atthe surface of the earth. For example, the source may provide 600 voltsat 400 cycles at each of its phases supplied via a three-pole,single-throw switch 101 and cable conductors 102, 103 and 104 todeltaconnected step-down transformers 105, 106 and 107 mounted withinhousing 13. By utilizing transformers having a suitable stepdown ratio,there is thus available at conductors 108, 109 and 110 a three-phasesupply at volts. It is obvious that by utilizing a voltage on the orderof 600 volts, for example, for power transmission in cable 17, at agiven power consumption, the power loss due to the resistance of thecable conductors is low, as compared with the loss for transmission at alower voltage, say 115 volts.

lOne phase of the supply current is applied over conductors 108, 109 toa conventional power supply 111 adapted to convert the appliedalternating voltage to a higher, unidirectional potential. The latterpotential is supplied over conductors 112 to the anode circuit (notshown) of a radio-frequency generator 113 (FIG. 1B) operating at afrequency in the range from 10 to 100 megacycles per second. Theradiofrequency source, in turn, is conected by conductors 115 and 116and leads 26 and 27 to electrodes 24 and 25 of ion source 21. Filamentpower for the radiofrequency generator 113 is supplied over leads 117 bya step-down transformer 118 energized from another phase of the supplycurrent available at conductors 109 and 110.

The remaining phase of supply current, available at leads 108 and 110,is fed via a voltage control circuit 119 (to be described more fullyhereinafter) and lead 120 Ito a high voltage power supply 121 (FIG. 1C).The power supply 121 may be of conventional construction, or may be ofthe type to be later described in connection with FIGS. 5-7. It providesa unidirectional potential between output leads 122` and 123 in theneighborhood of 100 kilovolts. Lead 123 is the positive terminal of thesupply and is grounded to housing 13, while lead 122, the negativeterminal, is connected to conductor 45 (FIG. 1B) which, as pointed outabove, is connected to the target electrode 41. Power supply 121 alsosupplies a lower voltage of about 11/2 kilovolts over leads 124 and 37tofocus electrode 39. The accelerating gap circuit is completed by aconnection between lead 36 of probe 33, 35 and housing 13.

In order to control the neutron flux produced by generator 14, adetector (FIG. 1B) in the form of a proportional counter -tube 125 isdisposed in housing 13 in the general vicinity of the target electrode.The detector is provided with a lining of hydrogenous material (notshown) on the side facing generator 14 and is filled with argon gas. Oneoutput lead of tube 125 is grounded to housing 13 by a connection 126and the remaining output lead is connected by a lead 127 to a couplingcondenser 128 (FIG. 1A), in turn, connected to a conventional pulse.amplifier and integrator unit 129. The necessary voltage for countertube 125 is supplied by a conventional power supply 130, energized frompower leads 109 and 110, over a lead 131 and a decoupling resistor 132.The power supply circuit is completed .by a connection 133 to housing13.

The output of unit 129, which is representative of a characteristic ofthe generated neutron llux, such as counts per unit time, is applied tothe input circuit of voltage control circuit 119 over a lead 134 andconnections 135 and 136 to housing 13. Circuit 119 may be ofconventional construction; for instance, it may include a magneticampli-fier connected in a servo circuit which compares the potentialfrom amplifier-integrator 129 with a reference potential to derive acontrol effect. This control effect may be the adjustment of the valueof an impedance effectively connected between leads 110 and 120.Accordingly, the potential which energizes high-voltage power supply 121is dependent upon the generated neutron flux so that this flux is.automatically maintained at a substantially constant value.

Amplified pulses from unit 129 are supplied via a conductor 137 whichextends through cable 17 to an indicator 138, such as anintegrator-voltmeter, of surface equipment 9. 'The indicator circuit iscompleted by .a connection 139 between housing 13 and shield 140 of thecable and a ground connection 141 at the earths surface between t-heshield and the indicator 138.

In order to energize the pressure-measuring device in envelope section20 (FIG. l-B), anode lead 51 (or 54) is connected to power supply 130`by .an extension of lead 131 through resistor 149 and jumper lead 152.Cathode leads 52 and 53 are tied together and are connected by aresistor 142 to housing 13. The resistance value of resistor 142 isselected, in a known manner, to counteract the negative resist-ancecharacteristics of the glow discharge between anodes 57, 61 and cathodes62, 63 of the pressure gauge as well as to derive a voltage representinganode-cathode current. Resistor 149 interposed in lead 131 is similarlyemployed to counteract the effect of the negative resistance of theionic discharge in ion source 21.

The junction of resistor 142 with the cathode leads is connected by .alead 143 to one input terminal of a pres- -sure control circuit 144,having its other input terminal connected by a lead 145 to housing 13.Circuit 144 may be o-f conventional construction comprising, forexample, a magnetic servo amplifier for comparing the potentialdeveloped across the resistor 142 with a reference po-tential to prevent.an impedance between output circuit leads 146 and 147 that isautomatically controlled by the difference between tlie developed andreference potentials.

Lead 146 is connected to supply lead 109 and lead 147 is connected toone terminal of the primary winding of a step-down transformer 148. Theremaining primary terminal is connected to supply lead 110. Thesecondary winding of transformer 148 is connected .by leads 150 and 151to leads 49 and 5t) of deuterium-emitting filaments 69, therebycompleting the automatic pressure control system. A connection 152between anode 57 of the pressure gauge and filament 69 is employed tomaintain these elements at the same potential in order to preclude thepossibility of an ionic discharge therebetween.

Since the potential developed at resistor 142 is a measure of thepressure in envelope 18, this potential is also supplied by a cableconductor 151 to an indicator 152', such as a voltmeter, of .surfaceequipment 9. If desired, a pressure-representing-potential derived incontrol circuit 144 may be utilized to actuate indicator 152'.

Lead 55 of getter filament 70 is connected to conductor 151 and itsother lead 56 is connected by a conductor 153 to another secondarywinding 154 of step-down transformer 118. A connection from thesecondary winding 154 to lead 151 completes the circuit. Thus, filament70 may be energized continuously during operation of the neutrongenerator.

A similar form of gas-pressure control system is disclosed in thecopending .application of Sidney Soloway, tiled Dec. 2l, 1953, bearingthe Ser. No. 399,505, and issued Mar. 3l, 1959, as Patent No. 2,880,373.

Power supply also provides a voltage for energizing Vunits 119, 129 and144. This voltage is supplied via a lead 155 and various extensionsthereof.

Tihe portion of the Well logging apparatus thus far described relates tothe generation of neutrons for irradiating formations 11. In order .toobtain a log, for example of the resulting gamma radiation, means areprovided for energizing detector 15, which may be a Geiger-Meuller tube(FIG. 1C), and for recording a characteristic of its output. To thisend, a source of alternating current in surface equipment 9 is coupledto a transformer 161 having one terminal of its secondary windingconnected to the groundedshield 149 and the other terminal connected viaan isolating choke 162 to a conductor 163 of cable 17. Conductor 163traverses housing 13 and is.

connected to the housing via the series-connected primary windings oftransformers 164 and 165. Transformer 164 is a power transformer for .aconventional power supply 166 having a connection 167 to housing 13.

Power supply 166 develops the high voltage for op erating tube 15 whichis applied thereto via an isolating resistor 168. The remaining terminalof tube 15 is connected by a lead 169 to housing 13. The junction ofresistor 16S with the lead to tube 15 is connected by a couplingcondenser 170 to the input circuit of a conventional pulse amplifier171.'The input circuit is completed by a connection 172 to the housingand .a voltage of suitable magnitude for operating the amplifier isderived from power supply 166 over a lead 173. Transformer 1.65 is apulse transformer to which the output signal of ampliiier 171 isapplied.

This ouput signal is derived at the surface equipment 9 by a pulsetransformer 174. The primary winding of the transformer is connected toa filter including a series condenser 175 and shunt choke 176 forattenuating voltages at the frequency of source 160. The transformerssecondary winding is connected to a conventional integrator andrecording unit 177. Unit 177, for example, may comprise a capacitor forderiving a potential representing the number of pulses applied per unittime and a recording voltmeter to which this potential is applied. Therecording medium ofthe voltmeter is displaced in a customary manner insynchronism with movement of housing 13 through bore hole 10 so that acontinuous log may be obtained.

In operation, housing 13 is lowered into bore hole 10 prior to theclosing of switch 101. Thus, operating personnel are shielded from anydangerous radiation emanating from neutron generator 14 by the earthformations 11 and drilling fluid 12.

With switch 101 closed, radiofrequency generator 113 is energized andits output is supplied to electrodes 24 and 25 of ion source 21. Inaddition, high voltage power supply 121 delivers its output voltage tothe accelerating .gap defined by the periphery of aperture 34 inelectrode 33 and layer 44 of target 41, as well as an intermediate orlower potential to focus electrode 39. Moreover, a positive potential issupplied by source 130 to anode 57, 61 relative to the cathodes 62, 63of the pressure gauge 20.

Although the potential between the electrodes of ion source 21 inassociation with the axial magnetic field created by magnet 28 areconducive to strong ionization of deuterium gas, this phenomena is acumulative process which must be initiated from an external source.Normally, in equipment operated at the surface of the earth,

the usual cosmic radiation present is sufficient to initiate adischarge. However, the generator 14 is shielded when in the bore hole;thus, radio-active material 29 is provided for this purpose.

Accordingly, initial ionization of the gas is followed by a continuousionic discharge in the radiofrequency field coupled to the gas viaelectrodes 24 and 25. Glass partition 19 serves to prevent therecombination of ions at the surface of magnet 57, glass cylinder 23prevents recombination at electrodes 24 and 25, while conical glassmember 31 prevents such recombination at the surface of probe 33.Therefore, ion source 21 operates more eiciently in producing ions fromthe gas present in envelope 13 than might otherwise be possible.

Cylinder 23 is constructed of glass thin enough so that theradiofrequency voltage drop across it, due to its high dielectricconstant, is low. This results in a higher radiofrequency voltage acrossthe gas in the ion source 21 and a more etfcient coupling, betweenoscillator 113 and the gas, than is otherwise possible.

Since probe electrode 33, 35 is at the potential of housing 13, whilemagnet 57 is at a positive potential relative thereto, ions in source 21tend to drift in a direction from the magnet toward the probe. Some ofthese ions pass through opening 32 in glass member 31 and opening 34 ofprobe 33 and are thus introduced into the accelerating gap deined by theprobe and the target 41, 43. Because of the high potential impressdbetween the probe and the'target, positive ions are accelerated to highvelocities prior to striking target material 44. The highly acceleratedpositive deuterium ions thus react with the tritium in target material44 and neutrons of energies at a level of 14 million electron volts aregenerated.

Inasmuch as the accelerated ions must pass through opening 40 in thefocus electrode 39, which is maintained at a negative potential relativeto probe 33, defocusing of the ion beam may be elected prior to itsirnpingement at target material 44. Accordingly, instead of a highlylocalized point of impingement, as might otherwise result, the ion beamis distributed over a larger area of the target, thereby preventinglocalized heating and burn-out of the target material.

Positive ions upon striking target material 44 produce secondaryelectrons which are accelerated across gap 44-33 in the directionopposite to positive ion travel. Most of these electrons pass throughopenings 34 and 32, traverse source 21 without collision and eventuallypass through opening 22 in partition 19. Such accelerated electronsimpinge upon baffle plate 68 which absorbs their energies by conversionto heat which is dissipated. Consequently, these electrons are preventedfrom striking magnet 57 where they might undesirably cause localizedheating and the occlusion of absorbed gases. In addition, plate 68serves to minimize recombinations of electrons and ions at the surfaceof magnet 57. Furthermore, since baie 68 is made of low atomic weightmaterials, only soft X-rays are produced by electron bombardment.

Filament 70 is heated by the current flowing through it to a temperaturein the neighborhood of 1200" C. At this temperature, the lament reactschemically with such gases as oxygen and nitrogen which may be presentas impurities in the tube, and absorbs them; but it does not absorbdeuterium.

High energy neutrons emanating from target material 44 irradiateformations 11 as well as detector 125. A small fraction of the fastneutrons incident on the detector produce recoil protons in thehydrogenous lining. Some of these protons cause ionization in the argonand the resulting pulses are amplified and integrated in stage 129 todevelop a control potential supplied to voltage control circuit 119. Ifthe neutron flux increases, the number of counts per second increasesand the voltage control circuit reduces the voltage supplied to highvoltage power supply 121. Accordingly, a lower voltage is applied to theaccelerating gap, thereby decreasing the neutron flux. Conversely, adecrease in the neutron flux causes an increase in the high voltagesupplied to the accelerating gap. In this way, the neutron yield remainssubstantially constant over a wide variety of operating conditions.Since only a small fraction of the neutrons are intercepted by thedetector, it is not overloaded even though it is relatively close to theintense source. Moreover, because of the small attennation in thedetector, the symmetry of the flux of fast neutrons incident on thesurrounding formations is not adversely affected.

In the pressure control system, positive ions are created in the polegap of magnet 57 by spiralling electrons which are emitted from cathodes62, 63 when positiveions strike these cathodes. Further electrons areemitted which, in turn, produce further positive ions and a continuousdischarge occurs. The resulting current is a function of the gaspressure, since that pressure determines the number of positive ionswhich can be produced. The potential developed across resistor 142controls pressure control circuit 144 which, in turn, adjusts the powerthat is supplied to deuterium-emitting filament 69. This filamentoperates in the temperature range of approximately 300 to 600 C. and thecontrol circuit is arranged so that the filament temperature isincreased when a decreased voltage at resistor 142 indicates a'decreasein pressure within envelope 18. Conversely, the lilament temperaturedecreases when the pressure in the envelope increases and pressure maythus be maintained constant at a desired value.

Of oourse, the circuit may be suitably arranged so that the temperatureof filament 69 may be reduced automatically to a temperature at which itabsorbs deuterium to compensate for an increase in pressure and, upon areduction in pressure, is returned to an emitting temperature.

Irradiation of the formations 11 by the high energy neutrons produced ingenerator 14, results in nuclear radiation that is incident onGeiger-Meuller tube 15. This occurs in a process wherein the neutronsare slowed to energy levels low enough to permit reactionsproducingcapture gamma rays. The detector responds to gamma rays and itsoutput is in the form of pulses which are amplified in stage 171 Ibeforebeing fed to the integrator and recording unit 177 of surface equipment9. It is therefore apparent that a neutron-gamma ray log is obtained.This log features information regarding the earth formations traversedby the bore hole, such as enumerated in detail in the copendingapplication of Clark Goodman, filed Mar. 11, 1952, bearing the Ser. No.275,932, and assigned to the same assignee as the present application. i

.Since automatic controls are provided for pressure and neutron iiux,the neutron generator embodying the present invention may be reliablyoperated during an entire logging run. The operator need not beconcerned with any critical and continuous adjustments to the equipment.

In well logging the variation in gamma radiation incident on detectorshould be indicative of changes in the surrounding formations 11 only.Variations in the neutron yield will, of course, produce changes in thegamma radiation received by 15. Hence, maintenance .of a constantneutron yield is an essential feature of this aspect of the presentinvention.

In general, by reason of the construction of generator 14, as evident inFIGS. 1B and 2, a relatively rugged device is possible. Moreover,generator 14 has a conguration and is small enough so that it is adaptedto the elongated, Asmall-diameter, cylindrical housings suitable to -bepassed through conventional oil eld bore holes. The remainder of thebore hole apparatus may be readily accommodated to the size andruggedness specifications of bore hole apparatus.

Therefore, the well logging apparatus embodying the present inventionmeets all requirements of logging service.

As an alternative to the control of neutron ux by adjusting the voltageapplied to the -accelerating gap, target layer 44 may be loaded withtritium so that the content of this gas in the target increases in aradial direction and a voltage control circuit,^sirnilar to unit 119, isprovided to control the voltage applied to focus electrode 39. Thesystem is arranged so that as the neutron ux decreases, the diameter ofthe ion beam incident on target layer 44 increases and vice versa. Thus,the neutron flux may be maintained at a selected, constant value. Inactual operation, a uniform target attains the desired tritiumdistribution in a short time.

Of course, if desired, the tritium content in layer 44 of the target maybe arranged so that it increases along a given path. In this case, apair of deilection elements, such as electrostatic plates, may beemployed in place of focus electrode 39. Thus, by adjusting -anotherdimensional characteristic of the beam of ions incident on the targetlayer, namely, its deflected position along the aforesaid given path,rather than its cross sectional area, the neutron ux output of generator14 may be controlled in a desired manner.

Instead of a iilling gas of deuteriu'm, a mixture of tritium anddeuterium may be employed in suitable proportions so as to provide themeans for constantly refilling target layer 44 with an equilibriumconcentration of each of these gases. For example, a mixture of 50%tritium and 50% deuterium has been determined as suitable, and thismixture may be used for hydriding gas-emitting lament 69.

In operation, the tritium and deuterium gases are ionized in source 21and ions of both are accelerated and impinge upon target layer 44. Thedeuterium ions are involved in the deuterium-tritium reaction forneutron generation, as described hereinbefore, while some of theaccelerated tritium ions are absorbed by the layer.

By thus continuously replacing tritium in the target layer 44, frequentreplacement of the target is avoided. Moreover, the expensive gastritium is conserved, as is evide-nt from the following discussion.

In the usual fabrication process for zirconium target layer 44, thelayer is much thicker than the depth of penetration of bom-barding ions;however, the entire layer absorbs tritium during the hydriding process.Consequently, only a fraction of the tritium enters into theneutron-producing reaction.

If a deuterium-tritium mixture is employed, the target layer may behydrided merely by operating the generator. Only a shallow surfaceportion of the layer 44, within range `of bombar-dng ions, is thusfilled with tritium and essentially none is unused when the generator isemployed to derive neutrons.

Of course, other types of logs may be derived. For example, detectortube 15 may be a .proportional counter lined with hydrogenous materialor a boron compound.

In that way, a neutron-neutron log may be obtained. Moreover, byproviding suitable detectors, both a neutrongamma ray and aneutron-neutron log may be obtained simultaneously.

1f desired, generator 14 may be pulsed and the detection systemassociated with tube 15 gated to achieve an activation log. For example,the power supply that provides the accelerating potential for generator14 may be arranged to deliver the high voltage in pulses, rather than ata constant value. Alternatively, a steady voltage may be applied to theaccelerating gap and pulses, of positive polarity'relative to probe 33,35, may be applied to focus electrode 39. Ineither case in which theneutron generator is pulsed, higher peak voltages may be employed-wit-'hout 'breakdown in the accelerating gap, as contrasted with anon-pulsed generator.

Pulsing may be accomplished by selectively energizing ion source 21. Tothis end, a conventional pulsing circuit may be associated withradio-frequency generator 113 so that the generator deliversradiofrequency energy to -the ion source during operating intervals ofselected duration. Accordingly, neutrons are generated only yduring suchintervals.

Another type of log which may be obtained with the system shown in FIGS.lA-C, requires that monitor be responsive to neutrons slowed by theformations to thermal energy levels. Preferably, the monitor ispositioned in the vicinity of detector 15 and may comprise a countertube lined with boron and lled with argon gas, or it may be unlined andlled with boron trifluoride gas.

In operation, the high energy neutrons from generator 14 are slowed bythe earth formations and the bore hole fluid to thermal levels and someare returned to detector 125. Inasmuch as the output of detector 125 isemployed in stage 129 to derive a control potential for operating thecontrol circuit 119, the high-energy neutron ux output of generator 14is adjusted in a manner maintaining the flux of returning thermalneutrons constant. Accordingly, the log of capture gamma rays detectedby counter 15 is representative of a condition of constant thermalneutron ilux. This log provides information regarding theneutroncapturing and gamma-ray production qualities of the zones underinvestigation and is relatively independent of formation porosity. Sucha log may be useful, for example, in correcting for the chlorine contentof formations of similar porosity.

This type of control of the generated neutron flux in response toreturning slow neutrons may also be employed in gamma-ray spectralanalysis. For such an application, a conventional gamma-ray spectrumanalyzer may be substituted for detector 15 and its associated circuits.

If a simultaneous thermal neutron log is desired, a fast neutron counterlike the one designated 125 in FIG. 1B may be connected to indicator138.

Of course, the monitor may be made responsive to epitherrnal neutrons.This may be accomplished by using a counter tube `for thermal neutrons,as described above, enclosed in a cadmium shield.

In FIG. 3, there is illustrated in longitudinal cross section a modifiedform of ionization gauge which may be employed in section 20 ofgenerator 14. It comprises a cylindrical magnet supported by leads 51and 54 Within, and in transverse relation, to envelope 18. Disc-likecathode plates 181 and 182 are arranged in parallel, spaced relationwithin the cylindrical magnet and are connected to leads 52 and 53,respectively, which pass through header 48 of envelope 18. The portionsof leads 52 and 53 within the envelope are coated with respectivesleeve-like layers 183 and 184 of glass insulation which are outwardlyto form funnel-like transition sections 185 and 186 connected tocylindrical sections 187 and 188. The latter sections, 187 and 188,isolate or shield the plates 181 and 182 from the inner wall surface ofmagnet 180, but are spaced from one another to ydefine an exposedannular surface portion 189. To communicate the space between cathodes181 and 182 with the gas in envelope 18, cylinder 180 is provided with aplurality of radiallyextending openings 190. Alternatively, the plates181, 182, and glass sections 187, 188 may be reduced in diameter forthis purpose.

In operation, the 'magnet 180 provides a magnetic eld component directedtransversely with respect to cathode plates 181 and 182. Annular portion189 of the magnet functions as an anode. Thus, the gauge performs inessentially the same manner as does the one illustrated in FIG. 2.

The neutron generator of FIG. 2 may be modified in the manner shown inFIG. 4 wherein the portion of the generator below partition 19 isrepresented in vlongitudinal cross section.

The modified neutron generator includes an ion source section 191comprised-of a cylindrical, wire helix or anode 191a connected to andsupported by a plurality of support rods 19117. A cathode .plate 191e`is supported at the upper end of helix 191a in coaxial alignment there-13 with by an L-shaped rod 191d which is secured to one of a pluralityof support rods 191e. Supports 191b and 191e extend longitudinallythrough envelope 18' and are suitably arranged to pass through partition19 (FIG. 1), to one side of the elements within envelope section 20, andthrough header 48.

The lower extremities of support rods 191e are connected to a flat,disc-shaped metallic electrode 191f which is spaced from the lowerextremity of helix 191a. Electrode 191f has its peripheral edgev191gflared upwardly and is provided with a central opening 191k, also havingan upwardly flared edge. A second cathode 191i having a central opening191]l is supported in the vicinity of the plane of the lower end ofhelix 191a by a wire helix 191k which extends upwardly from disc 191i.Electrodes 191f, 191i and 191k may be termed a probe assembly by meansof which ions are extracted from source 191.

A cylindrical magnet assembly 192 receives the portion of envelope 18within which ion source 191 is enclosed. The magnet assembly comprises aprincipal section 192a of rectangular cross section and upper and lowerauxiliary sections 192b and 192C of triangular cross section. Theauxiliary sections 192b and 192C are designed to minimize fringingeffects, thereby to provide, in connection with principal magnet section192er, a more uniform axial magnetic field for ion source 191 thanotherwise possible.

If desired, additional elements (not shown) of magnetic material may bedisposed within envelope 18 so as to shape or control the magnetic fieldand provide a required uniformity in the field between cathodes 191C and191i. l

The lower portion of envelope 18 is flared inwardly to form a reentrant,coaxial tube 193 which is fused to a cylindrical, metal connector 194 onwhich a coaxial target assembly 195 is supported. This assemblycomprises a cylindrical glass insulator 195a having its lower end fusedto the upper end of cylinder 194 and its upper end fused to the lowerend of a metal tube 195b. Metal tube 195b extends upwardly and isterminated by a flat target member 195e constructed of zirconium, forexample, which isl hydrided with the isotope tritium. A metal connector195d is welded within tube 195b at the upper extremity thereof, and isprovided wtih a threaded opening 195e.

The neutron generator of FIG. 4 also includes a suppressor electrodeassembly 196 comprised of a disc 196a having a central opening 196b forreceiving connector 194. Disc 196e is welded toconnector 194 and to aplurality of support rods 196e which extend upwardly through the lowerportion of envelope 18. Support rods 196e terminate at a metallicsuppressor electrode or disc 196d, having a downwardly flared peripheraledge 196e and a central opening 1961i The disc material about opening196f is flared downwardly in opposition to the fiaring of the edge ofopening 191/t to minimize the possibility of breakdown.

Suppressor electrode 196d is supported wtihin the envelope in a selectedposition between electrode 191;:c and target 195C. To minimize furtherthe possibility of breakdown, a portion 197 of envelope 18', enclosingthe area between electrodes 191f and 196d, is reduced to a diametersmaller than the diameter of these electrodes.

In the ion source 191, the electric field between anode 191a andcathodes 191e and 191i is such that it is crossed with the magneticfield produced by magnetic assembly 192, leading to circular electronpaths of long total length, and consequent high ionization. In the gapbetween electrodes 1911c and 196d, the electric and magnetic fields arenot crossed and ionization is reduced below the intensity at which adischarge is initiated. Moreover, the electrode spacings in the ionsource and in the accelerating gap are suitably arranged in connectionwith the foregoing considerations to provide the desired ionization inthe ion source and freedom from ionization in the acceleratin gap.

In order to complete electrical connections to electrodes e and 196d, aconnector assembly 198 is provided. This assembly comprises a metallicrod 198a which extends through envelope section 193, connector 194 andtubular members 19Sa and 19511. The upper end of this rod is threadedinto opening 195e and thus is electrically connected to target 195C. Aportion of rod 198a receives a sleeve 198b of an electrically insulatingmaterial having a shouldered upper end 198C and a threaded lower end198d. An expansible connector 198e of generally cylindricalconfiguration is supported on insulator 198b with its upper end abuttingshoulder 198C. Its lower end is engaged by a driving sleeve 1981, inturn, having its lower end engaged by a driving nut 198g.

Member 198e, for example, may be constructed of a cylinder of resilientmaterial having a plurality of slots extending longitudinally so thatwhen the cylinder is compressed in an axial direction, the materialbetween the slots is driven radially. Accordingly, when nut 198g istightened, cylinder 198f is driven upwardly and member 198e iscompressed between the cylinder and shoulder 198C, and it expands intoelectrical engagement with the inner surface of connector 194. Thus, anelectrical connection to suppressor 196d may be made by connecting alead 198k to nut 198g.

An electrical connection to target 195g may be made by extending a leadfrom rod 198a. However, in the illustrated arrangement, a droppingresistor 199 is connected between nut '198g and one of a plurality ofmetallic cooling fins 19941 that are fixed to the lower end of rod 198e.

An energizing circuit for anode 191e and cathodes 191e and 191i of ionsource 191 may be provided in a manner described in the copendingapplication of I. T. Dewan, filed Apr. 9, 1952, bearing the Ser. No.281,378, and issued Feb. 28, 1961, as Patent No. 2,973,444. This highvoltage for the accelerating gap of the generator may be supplied overconnections to one of support rods 191e and lead 198k.

In operation, deuterium gas withinenvelope 18 is ionized within ionsource 191 which operates in the manner described in the aforementionedapplication of J. T.

Dewan. Some of the resulting ions pass through opening 191] of cathode191i, helix 191k and opening 191k and thus come into the influence ofthe accelerating potential applied between probe plate 191f andelectrode 196d. Many of these ions are accelerated to high velocities,pass through opening 196f in suppressor 196d and strike target layer195e with a velocity suitable to the production of neutrons.

Since the accelerating gap current flows through resistor 199, -avoltage drop is developed which maintains suppressor 196d at a negativepotential relative to target 195g. Under this condition, secondaryelectrons which might otherwise be accelerated in a direction oppositeto ion travel are repelled toward the target and, for the most part, donot pass through opening 196]c of electrode 196d, Thus, electron currentflow within the neutron generator is minimized, thereby reducing thepower required for its operation.

Obviously, the relatively flat construction of electrodes 191f, 196d and195e in the neutron lgenerator -of FIG. 4 permits relatively simpleconstructional techniques to -be employed in the fabrication of aneutron generator embodying the invention. Accordingly, the cost of sucha generator is materially lower than other types.

In FIG. 5 of the drawings, there is illustrated one form of circuitwhich may be employed for high voltage power supply 121 of FIG. 1C.Input leads 108 and 120 are connected to the primary winding of astep-up transformer 200 which supplies 20 kilovolts to a voltagemultiplier of conventional construction employing diode rectifiers201-204 and charging condensers 205-208. The filaments of the dioderectifiers are energized by individual generators 209-212 which aredriven simultaneously by mechanical couplings of electrically insulatingmaterial, schematically illustrated by dash line 213. Motive power forthe generators is supplied by a motor 214 connected to power leads 108and 110.

The output of the voltage multiplier is derived between housingconnection 123 and lead 122. A pair of seriesconnected surge resistors215, 216 are interposed in lead 122, and a pair of series-connectedresistors 217, 218 form a voltage divider connected between the junctionof resistor 215 with condenser 208 and lead 123. 4Lead 124 is connectedto the junction of resistors 217 and 218 which are apportioned to.provide a potential of the order of 11/2 kilovolts between leads 123and 124.

In operation, the alternating potential at leads 108, 110 energizesmotor 214 which drives generators 209-212 at an essentially constantspeed. Thus, the filament potentials applied to rectifiers 201-204 areof substantially constant amplitude values. The alternating potentialbetween leads 108, 120 is stepped up to approximately 20 kilovolts bytransformer 200 before being multiplied by the system of rectifiers201-204 and condensers 205-208 to a unidirectional potential ofapproximately 100 kilovolts, in a well known fashion.

Since the potential at leads 108, 120 is under the control of voltagecontrol circuit 119, the output potential of the power supply availablea leads 122, 123 is automatically adjusted in the manner describedhereinbefore in connection with neutron flux control.

In FIG. 6 there is illustrated a typical physical arrange yment for thecomponents of the power supply shown in FIG. 5. These components, whichhave been assigned the same reference numerals used to identify theircounterparts in FIG. 5, are enclosed by -a cylindrical glass housin-g220 supported coaxially within housing 13 by upper and lower, flanged,resilient shock mounts 221 and 222. These shock mounts are associatedwith upper and lower retaining rings 223 and V224 which are secured tohousing 13 'by means not shown.

The housing is closed by upper and lower discs 225 and 226 suitablysealed to the inner surface of glass housing 220 so that a fluid-tightcontainer is achieved. For example, O-type sealing -rings 227 and 228may be employed for this purpose. A valved tube 229 passes through lowerclosure 226 and is employed Vfor admitting an oil filling into closedglass cylinder 220. If desired, another valved tube (not shown) may beprovided as an air vent to facilitate the filling process. A -centralopening 230 in lower closure 226 communicates with a closed bellows 231that accommodates changes in volume of the oil with variations intemperature.

Four support rods 232-235 extend longitudinally through cylindricalhousing 220. These rods are transversely spaced relative to one .anotherto define 4the corners of a square -figure in a horizontal plane, andthey are attached at their ends to closure plates 225 and 226. Theserods serve to space the plates 225 and 226 from one another, las well asto support a plurality of essentially similar chassis-like plates 236 inlongitudinally .spaced relation along housing 220.

The uppermost one of chassis plates 236 has mounted thereon rectifier204 and generator 212, as may -be seen in FIG. 7. Condensers 207 and 208are of cylindrical form and pass through respective openings 237 and 238in the uppermost chassis 236. They extend essentially like 'distancesabove and below this kchassis and are secured thereto by respectiveclamps 239 and 240.

Rectifier 203 and generator 211 are mounted on the next uppermost one ofchassis plates 236 (not s-hown). This plate does not require openingsfor receiving any of the condensers.

The third uppermost of plates 236 (not shown) supports rectifier 202 andgenerator 210 and has openings for receiving cylindrical condensers 205and 206. These condensers are so positioned that they pass throughopenings in the lowermost one of plates 236 which supports rectifier 201and generator 209.

As is evident in FIG. 6, the several .generators are mounted with theirshafts in axial alignment. They are mechanically connected together by aplurality of mechanical couplings 241 constructed -of an electricallyinsulating material suitable to withstand the highest voletagesdeveloped in the power supply. The lowermost of the couplings 241 isconnected to driving rnotor 214 that is supported below the lowermost ofmounting plates 236.

Transformer 200 is supported within glass housing 220 in a positionbelow generator 214. A plurality of leads 242 extend through, but areelectrically insulated from, closure plate 226 so that electricalconnections may be made to the transformer and to the motor. Of course,these connections are suitably sealed, in a well known manner, so thatthere may be no fluid leakage.

Resistors 215, 216 and 217 are mounted in the uppermost part of thehousing, while resistor 218 is mounted in the section containingtransformer 200. This disposition is provided because it is important toisolate the high voltage portion of the power supply from all otherpoints in order to prevent voltage breakdown.

A high voltage terminal 242 passes through a central opening in upperclosure 225. It comprises an electrically conductive rod 243 received bya sleeve 244 of electrically insulating material. The rod 243, thesleeve 244 and plate 225 are suitably sealed to effect a Huid-tightarrangement. Insulating sleeve 244 has a dielectric strength andthickness suitable to withstand the maximum voltage developed by thepower supply without breakdown. A plate 245 of insulating materialcovers the upper surface of closure 225 and is held in place by a ring246 of L-shaped cross section seated on retainer 223.

The physical arrangement just described features mechanical ruggednessand freedom from electrical breakdown. The power supply thus may operateefficiently and reliably in logging service.

The several generators are employed so that .electrical isolation may beachieved between the several lament circuits of the rectifiers whichobviously may be subjected to extremely high relative voltages in themultiplier circuit.

If battery operation of the rectifier filaments is desired, themodification of FIG. 8 may be employed. While the filament circuit forrectifier 201 alone has been shown, it is to be understood that theremaining rectifiers may be similarly arranged.

A filament battery 250 is connected to the filament of rectifier 201 viaa position-sensitive or gravity-operated switch 251. This switch may,for example, comprise a glass capsule 252 secured to glass housing 220.A pair of contacts 253 extend through the capsule wall and a Smallquantity or pool of mercury 254 is provided.

When the logging apparatus is in an inoperative, horizontal position,mercury pool 254 is out of engagement with contacts 253. However, whenthe apparatus is placed in a vertical position, required for a loggingrun, mercury pool 254 completes the filament circuit at contacts 253.

With this modification, the filament batteries may be enclosed withinoil-filled housing 220 and no undue cornplications in the filamentswitching circuit are required to accommodate the high voltages of thevoltage multiplier.

While particular embodiments of the present invention have been shownand described, it is apparent that changes and modifications may. bemade without departing from this invention in its broader aspects, andtherefore the aim in the appended claims is to cover all such changesand modifications as fall within the true spirit and scope of thisinvention.

What is claimed is:

1. A neutron source comprising an ion generator, a target ofgas-absorbing solid material, a gas supply containing at least two gasescharacterized by different atomic weights to provide gases for said iongenerator and said target to enable said generator to ionize said gasesand 1 7 fill said solid target with at least some of said gases, andthermally responsive means for replenishing said gases and establishinga substantially uniform pressure of said gases within the neutronsource.

2. A neutron source according to claim 1 wherein said gases comprise amixture of deuterium and tritium.

3. A neutron source comprising a target of gas-absorbing solid material,a gas supply containing at least two gases characterized by differentatomic Weights to provide gases for said solid target, thermallyresponsive means for replenishing said gases and for establishing asubstantially uniform pressure of said gases within the neutron source,means for producing an electrical eld, and means for producing amagnetic field, said electrical eld and said References Cited UNITEDSTATES PATENTS 2,689,918 9/1954 Youmans 250-84 2,712,081 6/1955 Fearonet al. Z50-83.6 2,908,823 10/1959 Ely Z50-84.5 X 2,926,271 2/ 1960Brinkerhoflc et al. Z50-84.5 X

ARCHIE R. BORCHELT, Primary Examiner.

3. A NEUTRON SOURCE COMPRISING A TARGET OF GAS-ABSORBING SOLID MATERIAL,A GAS SUPPLY CONTAINING AT LEAST TWO GASES CHARACTERIZED BY DIFFERENTATOMIC WEIGHTS TO PROVIDE GASES FOR SAID SOLID TARGET, THERMALLYRESPONSIVE MEANS FOR REPLENISHING SAID GASES AND FOR ESTABLISHING ASUBSTANTIALLY UNIFORM PRESSURE OF SAID GASES WITHIN THE NEUTRON SOURCE,MEANS FOR PRODUCING AN ELECTRICAL FIELD, AND MEANS FOR PRODUCING AMAGNETIC FIELD, SAID ELECTRICAL FIELD AND SAID MAGNETIC FIELD COACTINGTO IONIZE SAID GASES AT SAID UNIFORM PRESSURE AND THEREBY ENABLE ATLEAST SOME OF SAID GASES TO FILL SAID TARGET.